Two systems for the uptake of phosphate in Escherichia coli.

Mutants of Escherichia coli K-12 were constructed such that each possessed one single major system for phosphate transport. A comparison of these strains showed that one of the systems (PIT) was fully constitutive, required no binding protein, and operated in spheroplasts. It permitted the complete exchange of intracellular phosphate with extracellular phosphate (or arsenate) and was completely inhibited by uncouplers. The other system, PST, was repressible by phosphate concentrations above 1 mM, required the phosphate-binding protein for full activity, and did not operate in spheroplasts. It catalyzed very little exchange between internal and external phosphate and was resistant to uncouplers. The maximal velocities attained by the two systems were approximately the same, but the affinity for phosphate in the PST system was greater by two orders of magnitude. In strains in which both systems were fully operative, the initial rates of uptake was nearly additive, and the systems appeared to interact with a common intracellular phosphate pool.